Known types of apparatus are found to operate at relatively high loads of typically up to 1400 N. A consequence of these high loads is that the apparatus must be made sufficiently sturdy to carry the loads and drive the specimens against the resulting high frictional forces. This in turn means that the oscillating components of the apparatus must be relatively heavy. Having relatively large masses oscillating at frequencies of more than a few Hertz requires generation of substantial inertial forces which must react against the body of the apparatus. These inertial forces give rise to unwanted vibrations in the body of the apparatus which interfere with the accurate measurement of the frictional forces between the specimens.
A previous disclosure UK patent No 2162953, describes a method of reducing this unwanted vibration in the force measuring system by vibration isolation means. However, since the magnitude of the unwanted vibration depends only on the frequency and displacement amplitude of the oscillating masses and not on the load, when tests are carried out at low loads even very small unwanted vibrations can completely swamp the frictional forces of interest.
Another serious problem with known devices is that they give rise to problems of stroke length consistency under conditions of varying friction coefficient between the test specimens, especially at stroke lengths of less than 0.1 mm. The frictional forces generated in a sliding contact are inherently non-linear with respect to the sliding velocity or displacement. This means that at short stroke lengths there is a tendency for stick-slip behavior to occur, which can be difficult to control. Known apparatus which use mechanical linkages to drive the moving specimen do not suffer from this problem but cumulative tolerance errors in the linkage mean that accurate short stroke lengths are difficult to achieve with good reproducibility.